In a typical hard disk drive (HDD), servo sectors on the disk are used to provide position information about the location of a magnetic head over a disk surface. A common approach for writing such servo information on one or more disk surfaces in an HDD is referred to as spiral-based self-servo writing, or spiral-based SSW. According to this approach, multiple spiral-shaped servo information patterns (or “servo spirals”) are written on at least one disk surface prior to the SSW process. During the SSW process, a magnetic head of the HDD is positioned relative to a disk surface based on the servo spirals, so that the final servo information (the servo sectors) can be written on the disk surface by the magnetic head.
For an error-free and robust SSW process, the servo spirals used should be precisely written on the disk surface with a predetermined and constant slope. Such servo spirals may be written on the disk surface with an external media writer before assembly of the disk drive, or with a servo writing machine that uses an external precision actuator to position the disk drive actuator with a mechanical push pin through an opening in the disk drive housing. In either case, setup and use of such external equipment for each individual HDD is time-consuming and expensive in the context of high-volume manufacturing.
In light of this, in-drive spiral-writing schemes have been employed, in which an HDD itself writes servo spirals prior to performing the SSW process. For example, a set of coarsely positioned spirals may be written by the HDD while the actuator is moved across a disk surface by applying a suitable open-loop voltage profile, or by using velocity control that is based on back electromotive force (back-EMF) feedback. More precisely positioned spirals may then be written by demodulating signals from the coarsely positioned spirals.
However, because in-drive spiral writing methods can have significant velocity variation while individual spirals are written, differences in spiral-to-spiral spacing are common. In fact, neighboring spirals written by in-drive spiral writing methods can be located so closely to each other that, during subsequent demodulation of the servo spirals, the servo interrupt service routine for demodulating the spirals may not have sufficient time, after processing data for the first spiral, to process data for the second spiral, thereby skipping the timing and position information provided by the second spiral. In addition, neighboring spirals may cross, so that during demodulation a subsequent spiral may be read before an antecedent spiral. In either case, such loss or scrambling of servo information can result in loss of synchronization between firmware spiral numbers and spiral data tables storing data for each of the spirals written on the disk surface. Such issues during servo spiral demodulation can require rework or scrapping of the HDD.